1. Field of the Invention
The present invention relates to a method of developing a latent electrostatic image used for the electrophotography, the electrostatic recording, and the electrostatic printing.
2. Background of the Invention
Methods of electrophotographic development are divided into two groups, namely, so called a one-component developer method using toner as the main component and a two-component developer method using a mixture of toner and carrier such as glass beads, magnetic carrier, or their coated with a resin
As two-component developer method relies on the use of carrier for increasing the charged area for the toner, they are more stable in the charging properties than the most one-component developer method and thus favorable for ensuring the reproduction of high quality images in a long-run operation. Also, the two-component developer method is high in the toner feeding capability to a developing area and can hence be incorporated into high-speed apparatuses.
Such a two-component developer method is commonly employed in the digital electrophotography where a latent electrostatic image is printed on a photosensitive member with laser beam or the like and developed to a visible image.
Also, to cope with the decrease in size of the minimum unit area (a dot or pixel) of latent image while the increase in the density for improving the resolution, the reproducibility of highlight, and the color quality, various modifications of the method have been proposed with respect to processing conditions and developers (toner and carrier).
With regard to the two-component developer methods, in the during development, where assuming the traveling speed (mm/sec) of photosensitive member is Vp (sec) and the width of the image development area (the contacted width of the photosensitive member with the developer) is L (mm), a period of the time during a latent image being held in direct contact with a developer (=a developing period) is represented by An expression L/Vp (sec), if the L is smaller and the Vp is bigger, the developing period becomes shorter. And this shorten developing time declines the degree of development, thus causing undesired decrease of image density, non-uniform density in half toned image, making trace mark of developing brush, causing cutoffs in fine lines in image, forming white voids(blanks) of small size of dots in image and the like, thus deteriorating the quality of reproduced image.
For eliminating above mentioned drawbacks, a technique was introduced which included, for example, means for elevating the electric voltage of the photosensitive member to re the developing electric-potential and means for increasing traveling speed Vr mm/sec) of a developing sleeve so as to coincide with traveling speed Vp (sec/mm) of a photosensitive member moving in the same direction to bring in the more amount of developer to expand the contacting area of the developer with the latent electrostatic image. The rise of developing electric-potential of the photosensitive member is however suffered from an abundant electric charge passing through thereto, thus causing shortening of the life of the photosensitive member, therefore generally adopted means for overcoming the problem are those for increasing the amount of developer to be contacted.
Although an increased amount of developer to be contact by mean of using a difference between rotation speeds of developing sleeve and photosensitive member results in general a higher density of solid image, however the change in optical density and the occurrence of white voids are also very noticeable, especially at edge regions of solid image area and half toned image area. Such phenomenon appears at the area where the latent electric potential is varied sharply and discontinuously.
When the value of the Vr/Vp is greater than 1 with the photosensitive member rotating in the same direction as of the developing sleeve (referred to as forward rotation hereinafter), the carrier is traveling so as to outrun the latent electrostatic image which is also traveling.
Accordingly, at boundary region where the latent electrostatic image varies from background part to image part, developer arrives the background part before it enters into the solid part of image, thereby the toner particles held in carriers remain shifted (repelled) to the developing sleeve at the side opposite to the background part of the latent electrostatic image, by the effect of an electric potential equal to VBxe2x88x92VD, (where the VBis the biased direct-current and the VD is the charge potential).
Therefore, when the Vr/Vp is considerably greater than 1, the developer may fail to rapidly feed toner particles to the boundary between the background region and the solid image region, thus generating a white voids(blanks) in the trailing end (rear end of the latent image advancing forward) of the solid region.
During the developer is passing through the background region, its toner particles remain shifted to the sleeve side and less contacted to the photosensitive member. It may say additionally that this phenomenon (shifting of toner particles to tile sleeve side) will contribute to the protection from smears of the background.
As developer arrives from the background region to the trailing end of an image region, the developing area is now going to transfer the toner particles to the latent image for developing it by the effect of a developing potential (VLxe2x88x92VB, where the VL is the post-exposure potential and the VB is the biased direct-current potential), however on the time, the toner particles may hardly be supplied to be transferred, because they having been drifted to the sleeve side.
As a result, a more number of white voids will appear at trailing end of the halftone image area than at trailing end of the solid image area This can be explained by the developing electric-potential is a lower level at the half-tone region. It is now noted that the white voids(blanks) in the solid image are referred to as solid trailing end blanks and the white voids in the half-tone image are referred to as half-tone trailing end blanks hereinafter.
When the photosensitive member and the developing sleeve rotate in opposite directions (referred to as reverse rotation hereinafter), the foregoing phenomena may create blanks at the boundary between a background region and a solid region. The reverse rotation, unlike the forward rotation, permits the blanks in the leading end of the solid image.
Also, when Vr/Vp is smaller than 1 with the forward rotation, the carrier moves towards the latent electrostatic image hence generating a state resemble to the reverse rotation state and causing the blanks to appear in the leading end of the solid image.
For eliminating declinations in the image quality derived from the difference of the developing direction, some attempts were proposed which minimize the difference in the speed between the photosensitive member and the developing sleeve, however they were hard to give success. When the difference in the speeds is minimized, the image density may be declined or the smears of the background area may be generated. It is hence unsuccessful to provide a two-component developer method which can satisfy the both requirements of eliminating blanks and smears.
While digital technologies have been significantly developed for improving the image quality in recent years, the drawbacks pertinent to the developing direction(where the traveling speed of the developing sleeve is faster than that of the latent electrostatic image) may include not only the trailing end blanks in the developed image but also cutouts of the horizontal line, thickening of the vertical line, fault in the sharpness of characters (thickened in the vertical and thinned in the horizontal), and carrier deposition. It is hence desired to provide a further improvement of the method.
It is An object of the present invention to provide a developing method which can eliminate any undesired artifacts in the developed image derived from the developing direction(where the traveling speed of the developing sleeve is faster than that of the latent electrostatic image).
More specifically, the object of the present invention is to dissolve the undesired artifacts to be eliminated for developing a high-density image, which artifacts are: 1. trailing end blank; 2. cutout in the horizontal line; 3. thickening of the vertical line; 4. fault in the sharpness of characters (thickened in the vertical and thinned in the horizontal); 5. carrier deposition; and 6. smear of the background.
We, the inventors, have found through perpetual experiments the following aspects for achievement of the above and other objects.
1. With regard to trailing end blank and,
2. cutouts in the horizontal line
The above two undesired artifacts result from the fi t that the toner particles are drifted from the photosensitive member to the developing sleeve during the developing processing by the effect of an electric potential equal to VBxe2x88x92VD(where the VB is the biased direct-current and the VD is the charge potential) and thus decreasing the amount of toners on the surface of the photosensitive member. Also, it results as the toner particles are having been drifted, on the carriers may retain counter charges. When resin coated carrier is used for increasing the operating life of the developer and improving the image quality, it will heavily be affected by the counter charge.
It is hence essential to avoid such toner drift from the carrier surface. Also, desired is an improved developing system which allows the toner particles drifted to be returned back to the carrier surface immediately in response to a shift in the developing electric field.
Although the carrier is decreased in the density to meet with the magnetic brushing effect, it is found that the low-density carrier is not adequate for achievement of the objects. Alternatively, the carrier is attempted to decrease its bulk density relative to the real density for minimizing the concentration of the carrier in the mixture on a magnetic brush in the development stage. It is found that when the density of the GP agent is set to a particular rate, the distance between the carrier particles in the magnetic brush becomes favorable to enhance the movement (dispersion) of the carrier and thus discourage the drifting of the toner particles. More specifically, the crucial requirements for allowing the toner particles to be promptly transferred to the developing surface are realized by both determination of the adequate distance between the carrier particles and establishment of the easy movement of the carrier.
This allows the magnetic brush to avoid in thickened state, unlike that of the prior art, and hardly disturb the movement of the toner particles. Thus movement of the toner particles is significantly improved in the depth direction of the developer. It is also ascertained that the toner particles when drifted are readily effected by the developing potential thus to contribute to the development creating no printing smears in the solid image at the trailing end and the halftone image at the training end.
As the density of the developer is appropriated, its toner particles once deposited may scarcely be scraped off (scavenged) in both the solid and halftone images at the trailing end.
It is furthermore found through the experiments that when the carrier particles are arranged of a smaller diameter with the density of the GP agent set to a desired rate, their surface area becomes increased and permits the toner particles to be sufficiently charged to minimize the production of low charged or reverse charged particles and increase the margin for smear of the background, thus controlling the average charge of each toner particle to a low level, enriching the image density, and improving the image quality in relation to the developing direction. Also, the carrier with a smaller diameter permits the magnetic brush to be thick at the head and smooth in the movement hence creating less brushing traces.
Since the small diameter carrier of the prior art is low in the margin for carrier deposition, it may produce scratched trace on the photosensitive member or the firing roll thus actual use is difficult. It is found that when the carrier particles exhibit a specific pattern of diameters distribution, the drawbacks pertinent to the developing direction and the carrier deposition can simultaneously be eliminated.
3. With regard to thickening of Vertical Line
The vertical line may be thickened by the toner particles received from the (sleeve lengthwise) direction perpendicular to the traveling direction of the developing sleeve.
It is found that when the density of the GP agent is decreased in the developing area, the magnetic brush can be thinned to decline the feed of the toner particles from the horizontal direction at the proximity of the vertical line thus significantly inhibiting the thickening of the vertical line. As the carrier particle diameter is also small, the magnetic brush becomes uniform and relatively thick hence contributing to the inhibition of the thickening and undulation of the vertical lines.
4. With regard to the sharpness of character (thickened in vertical lines and thinned in horizontal lines)
Each character consists of more or less of horizontal and vertical lines and its sharpness (thickened in vertical lines and thinned in horizontal lines) depends on a combination of the three artifacts denoted in the above items 1, 2, and 3.
When the three artifacts are balanced, the sharpness can be improved with the carrier reduced in the particle size.
5. With regard to the carrier deposition
In the developing process of a stationary magnet type, the developer (toner and carrier) is equally oriented to the photosensitive member at the developing area. Therefore, as the developer arrives from a background region to a solid region of the latent image, it is effected by the an electric potential equal to VBxe2x88x92VD until entering into the solid region. The toner particles in the developer are biased to the developing sleeve and held less at the top of the magnetic brush, thus the carriers positioned in this head are charged at the reverse polarity. This causes the carrier deposition in a specific area such as the edge of a solid image where the electric field is reversed. When heavily effected by the potential of background area, the developer may gradually be drifted towards the developing sleeve. Upon departing from the developing area, the developer is charged (or counter-charged) at the polarity opposite to that of the toner. As a result, the carrier stays free from the force of magnetic flu: and may be deposited to the photosensitive member (as similar to development).
In a type of simultaneous rotations of both magnet and sleeve, as the carrier is continuously rotated at the developing area and toner does not liberalize from carrier, thereby on the carrier, counter charge to charge of toner is not resulted. As the carrier is substantially charged at no reverse polarity thus to create less white voids (blanks) in the half-tone image at the trailing end, the reproductive of horizontal lines can be improved. It may be estimated from the action of carrier-deposition mechanism that the toner particles on the carrier are not drifted to the developing sleeve but readily transferred to the latent image (thus allowing no delay in the developing process).
It is however necessary in the simultaneous magnet/sleeve rotation type to rotate the magnet at a high speed in response to the linear speed of the developing sleeve and the overall arrangement of the developing system will be complicated. For allowing the magnetic brush to come uniformly into direct contact with the photosensitive member, the magnet has to pass at least two or more polarities dug the latent image is positioned at the developing area. Even if the magnet has some dozen poles 7 the rotation at a speed higher than 1000 rpm win be needed This may generate mechanical vibrations, jitters, and heating up of the sleeve by eddy current, thus declining the quality of the developer and discouraging the achievement of the objects.
The present invention appropriates the magnetic brush density, the carrier particle diameter, and the magnetic properties at the developing area to decline carrier deposition. The higher the charge, the higher the counter-charge becomes. Accordingly, the toner charge per mass has to be determined to an appropriate level.
As described, the artifact by the developing direction can be overcome by appropriating the density of the GP agent and the carrier particle size. As the carrier has a desired pattern of particle size distribution, the margin for carrier deposition can be improved.
6. With regard to the achievement of less smear at background with improved the image density
Heretofore, if the amount of scooped up feed is sharply decreased, the optical density of image as well as the margin for smear of the background may be declined. It is found that the developing efficiency of the toner in the developer is significantly increased by controllably determining the density of the GP agent to a desired level and simultaneously, using the carrier having increased surface area and an unique pattern of particle diameters distribution. Accordingly; the developing method having a constitution specified below can be fee from both the undesired artifacts of smear of the background and of the developing direction
Namely, based on the foregoing aspects and results of analysis, the abovementioned and other objects of the present invention are achieved by the of methods according to the present invention featuring as denoted below:
(1) A method of developing a latent electrostatic image using a two-component developer system, having a ratio (Vr/Vp) ranges 1.2 less than (Vr/Vp) less than 3 where the (Vp) is a linear speed (Vp)[m/sec] of a photosensitive member and the (Vr) is a linear speed (Vr) [m/sec] of a developing sleeve, and applying a biased direct-current (VB) [by volt], wherein; a developing gap(Gp)[cm] as a distance at the nearest point between a photosensitive member and a developing sleeve is less than or equal to 0.6 mm, a ratio(xcfx81p/xcfx81a) satisfies an expression (xcfx81p/xcfx81a) less than 0.7 where the (xcfx81p) is a density[g cm3] of a developer at the nearest point between a photosensitive member and a developing sleeve, which is represented by an equation xcfx81p=J/Gp where J is an amount of developer scooped up (the (xcfx81p) is also described as xe2x80x9cthe density of the developerxe2x80x9d or xe2x80x9cthe density of GP agentxe2x80x9d in the specification) and the (xcfx81a) is a bulk density[g/cm3] of the developer, a carrier for electrophotography is used, the carrier is made of a carrier core particles having a weight average particle diameter(Dv) ranging from 25 xcexcm to 45 xcexcm, the particles of smaller than 44 xcexcm are more than or equal to 70 percent by weight, the particles of smaller than 22 xcexcm are less than or equal to 7 percent by weight, a ratio (Dv/Dp) between the weight average particle diameter (Dv )and the number average particle diameter(Dp) satisfies an expression 1xe2x89xa6(Dv/Dp)xe2x89xa61.30, the core particles are used by coated form with a resin material:
(2) A method of developing a latent electrostatic image using a two-component developer system according to paragraph (1), wherein; the core carriers have a magnetic moment (at one K Oe=1000 Oe) ranging 60 to 100 emu/g.
(3) A method of developing a latent electrostatic image using a two-component developer system according to paragraphs (1) or (2), wherein; a developing potential less than or equal to 350 volts is applied where the developing potential is defined by an expression (VLxe2x88x92VB) while the VL is a post-exposure potential and the VB is a biased direct-current potential;
(4) A method of developing a latent electrostatic image using a two-component developer system according to any one paragraph consisting of group of paragraphs (1) to (3), wherein; a potential (equal to VBxe2x88x92VD) of background area is less than or equal to 250 volts where the potential of background area defined by an expression VBxe2x88x92VD while the VB is a biased direct-current potential and the VD is a charged potential.
The density (xcfx81p) of the GP agent is equal to J/Gp (G/cm3) while Gp can be measured with the use of a thickness gage, laser beam, or the like.
The four features of the present invention for improvement abovementioned items of artifacts 1 to 6 of the developing process will now be described in the form of achieving means.
In the two-component developer system using a biased direct-current (VB) to be applied, as described above, it is essential that the distance (Gp, a developing gap) at the nearest point between the photosensitive member and the developing sleeve is not greater than 0.6 mm and established ratio is (xcfx81p/xcfx81a) less than 0.7 when xcfx81p is the density of the developer at the nearest point and xcfx81a is the bulk density of the developer. Also, the electronic photography carrier is used which is made of carrier cores having a weight average size ranging from 25 xcexcm to 45 xcexcm, in which the particles of smaller than 44 xcexcm are not lower than 70 percent by weight and the particles of smaller than 22 xcexcm are not higher than 7 percent by weight and the ratio between the weight average particle diameter Dv and the number average particle diameter Dp is 1xe2x89xa6(Dv/Dp)xe2x89xa61.30, the carrier are coated with a resin material; wherein;
xcfx81p =J/Gp[g/cm2] (referred to as the density of the developer or the density of GP agent hereinafter)
Gp=developing gap[cm]
J=amount scooped up feed[g/cm2]
xcfx81a=bulk density of the developer[g/cm 3]
Vr=linear speed of developing sleeve [m/sec]
Vp=linear speed of photosensitive member [m/sec]
VB=biased direct-current[volt]
Dv=weight average particle diameter[xcexcm]
Dp=number average particle diameter [xcexcm].
This method is a reverse of the prior art which intends to feed a large amount of the developer to the developing area for increasing the image density and avoiding undesired white voids(blanks) in the developed image.
Favorable range of the developing gap( Gp ) is less than or equal to 0.6 mm, more preferably less than or equal to 0.5 mm. When exceeding 0.6 mm, high enough optical density of image is hardly obtained, high excess density at periphery of solid image(namely strongly edge-effected image) and deposition of carriers near fringe of solid image are may conducted.
The scooped up feed J (g/cm2) is a density expressed by grams per square centimeter, of the developer amount given by stirring for 60 seconds in the developing sleeve run at a given processing speed then forcibly stopping the movement of the system so as make the developer passed through to a doctor blade and stayed at an area before fed into the developing area.
The (xcfx81p/xcfx81a) is a ratio of density( xcfx81p ) of developer or GP agent against for bulk density( xcfx81a )of the developer used, and is an indicator showing filling degree of developer at developing area. The (xcfx81p/xcfx81a) is density/density, therefore has the unit of no dimension. When the (xcfx81p/xcfx81a) is small, there are provided many spaces between carrier particles at developing area, thereby movement of toners are not impeded, thus causing a conscientious adhesion of toners for latent image. On the other hand, the larger (xcfx81p/xcfx81a) results the lesser space, therefore toners located at developing sleeve side distant from latent image are impeded for the movement by the dense magnet blush, thus causing a not conscientious adhesion of toners for latent image, while significant white voids or blanks at trailing end of trailing end of the solid image area and at trailing end of the halftone image area
Thus the reason why the (xcfx81p/xcfx81a) value has to be smaller than 0.7 in accordance to the present invention is relied on a purpose for improving white voids or blanks at trailing end of trailing end of the solid image area, white voids or blanks at trailing end of the halftone image area, and sharpness of image. On the other hand, the smaller (xcfx81p/xcfx81a) makes the lower optical density of the image. The lowering in optical density of the image may compensate by increase of linear speed of developing sleeve, however it also gives bigger centrifugal effect to the developer, thereby increasing a of toners, making apparatus dirty and spurring background significantly, accordingly, the linear speed of developing sleeve can not increase extremely. Another, the optical density of the image can be enhanced by elevating the developing electric-potential. However, the elevation of the developing electric-potential also causes an intensified electric field at periphery of solid image(namely strongly edge-effected electric field ), thereby effecting unfavorable white voids or blanks at trailing end of the solid image area, deposition of carriers near fringe of solid image.
Accordingly, upon consideration of development conditions for yielding a high quality image, although the lower limit of the (xcfx81p/xcfx81a) value is hard to decide facilely, however in the range of less than 3.5 in near speed of developing sleeve with less than 450 volts in developing electric-potential, more than 0.25 of the (xcfx81p/xcfx81a) value is favorable, and more than 0.30 of the (xcfx81p/xcfx81a) value is more favorable.
Using a bulk specific weight meter conforming to JIS-Z2504, the bulk density (xcfx81a) of the developer is calculated by filling a 25-cm3 stainless steel cup with 85xc2x15 g of the developer, removing an overflow of the developer with a flat stainless steel strip of 10 mm wide, and dividing the weight of the developer in the cut by 25 cm3.
The bulk density of the developer herein means the average toner concentration in the developer during the running action under given processing conditions.
The linear speed ratio (Vr/Vp) between the speed (Vp) of the photosensitive member and the speed (Vr) of the developing sleeve is preferably 1 less than (Vr/Vp) less than 3.5 and more preferably 1.2 less than (Vr/Vp) less than 3, where the Vr is the linear speed of the developing sleeve measured in m/sec and the Vp is the linear speed of the photosensitive member measured in m/sec. If the linear speed ratio (Vr/Vp) is less than 1, the amount of developer passing through latent image is decreased, therefore enough optical density is hardly obtained, and the cleaning effect in background area by magnet blush becomes few, therefore is likely to make background dirty. On the other hand, when more than or equal to 3.5, high optical density may obtain, but frying of toners moreover frying developers are increased, due to a strengthened centrifugal force for toners and developers, thus making apparatus dirty and smearing background significantly.